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v                                     EGGS   OF   PLATYPUS                                 IO7
of eggs, or at least ovoviviparity, would follow from the struc-
ture of the egg, since the abundance of yolk would do away
with the necessity for a placenta. That the eggs had this
Saurian characteristic was first definitely made known by Pro-
fessor Poulton -1 for Ornithorhynclius, and his results were con-
firmed later for JZchidna.2 The structure of the eggs has.,
however, already been dealt with on p. 72. The fact that
these animals lay eggs appears to have been known for a very
long time, though rediscovered so lately as 1884 by Mr.
Caldwell.3 In connexion with the structure of the ova, the
ovaries themselves and the oviducts are built upon the Saurop-
sidan plan. In the male the testes retain the primitive ab-
dominal position. The fact that the urinary and genital
products escape by means of their ducts into a chamber which
also receives the end of the alimentary tract is not a distinctive
feature of this group, inasmuch as it is seen in the Marsupials,
and also in certain low Eutheria, such as the Beaver and other
Rodents, and a few Insect ivores. As to external features, the
Monotremata show certain archaic characters. The unspecialised
arrangement of the mammary glands has already been described.
These animals are plantigrade, if the term may be used also to
describe the aquatic Orn-ithorJiyneJius. The ears are absolutely
destitute of a conch. The remarkable spur upon the hind-legs
furnished with a gland, which is more marked in the male, and
indeed disappears in the female of Ornitliorliynclius, is a structure
which argues the specialised condition of these two modern
representatives of what must have been a large order in the past.
The skeleton shows numerous ancient characteristics. In
the skull there is no demarcation of the orbit from the temporal
fossa, a feature widely found in archaic mammals. The tympanic
remains as a slender ring, there being 110 auditory bulla formed
either from, this or from any other bone. The mallexis and
incus are large, and thus reminiscent of the quadrate and
articular bone of reptiles. In the lower jaw the absence of
a marked coronoid process, and the absence of a firm ossification
at the meeting of the two rami, may be a primitive state of
1  Quart. Jbwrn. Micr. Sci. xxiv. 1884, p. 124.
2  Beddard, Proc. JRoy, Phys. Soc. JEditib. via. 1885, p. 354.
3  See Phil. Trans* clxxviil. 1887, where the literature of the subject is fully